Panels of gypsum wallboard which comprise a core of set gypsum sandwiched between two sheets of facing paper have long been used as structural members in the fabrication of buildings where the panels are used to form the partitions or walls of rooms, elevator shafts, stairwells, ceilings and the like.
In efforts to mitigate or overcome problems associated with the use of paper-faced gypsum wallboard in applications where moisture exposure is expected to occur, the prior art has approached the problem in various ways over the years.
One approach to the problem has been to treat the paper comprising the facing of the wallboard with a water-resistant material sometimes referred to as a water-repellant. Polyethylene emulsion is an example of a material that is used to treat paper facing to impart water-resistant characteristics. Such treatment is designed to deter delamination of the multi-ply paper facing by reducing the tendency of the paper to absorb water which is a chief cause of delamination and to deter water from penetrating through the paper to the gypsum and destroying the bond between the paper-facing and gypsum core.
Another approach to the problem has involved incorporating into the formulation from which the gypsum core is made a material that functions to impart improved water-resistant properties to the set gypsum core itself. Such an additive tends to reduce the water-absorbing tendency of the core and decrease the solubility characteristics of the set gypsum. Wax-asphalt emulsions and wax emulsions are examples of such additives.
Although improvements had been realized by the provision of gypsum wallboard prepared in accordance with these teachings, further improvements were still possible. Experience showed that even with such constructions the paper facing delaminated and the gypsum core eroded through the degrading action of moisture. The problem was particularly aggravated by warm water acting upon a gypsum core that includes either a wax emulsion or a wax-asphalt emulsion, commonly used water-resistant core additives. While cores containing such materials have relatively good water-resistant characteristics in the presence of water at room temperature, such characteristics start to fall off at temperatures in excess of 70° F. and tend to disappear in the presence of water having a temperature of about 100° F. or higher.
In another commercially successful approach, a structural panel comprising a set gypsum core sandwiched between two porous fibrous mats is provided, see U.S. Pat. No. 4,647,496. The preferred form of mat is described as a non-woven glass fiber mat formed from fiberglass filaments oriented in random pattern and bound together with a resin binder. Such panels differ from conventional gypsum wallboard in that the fibrous mat is substituted for paper as the facing material(s) of the gypsum core. In such constructions, the set gypsum from the core extends at least part-way into the fibrous mat facer to form an integral attachment/bond between the gypsum and the mat. In this construction the flow of gypsum into the mat during preparation of the board is unimpeded. Since the mat is completely porous, the gypsum flows freely into the pores (interstices) of the mat forming a strong bond with the mat.
The strength of the bond between the gypsum core and the fibrous facing is a critical factor in the usefulness of the panel in applications that require the attachment of tiles, insulating material (foam insulation) or other exterior finishing materials onto the fibrous surface. If the bond does not have a sufficient strength, the board is not strong enough to satisfy its supporting function. In that instance, the face sheet delaminates from the gypsum core. Experience has shown that the bond must have a tensile strength of at least about 16 psi, measured by the bond strength test described in detail below, in order to adequately provide the required supporting function.
Extensive outdoor testing has shown that glass mat-faced, water-resistant gypsum board of the type described in the aforementioned '496 patent has much better weathering characteristics, including water-resistant characteristics, than water-resistant gypsum board covered with water-resistant paper facings.
In a more recent improvement of this technology, as described in U.S. Pat. No. 5,397,631, the fibrous mat-faced gypsum board following initial board preparation is coated with a latex polymer. The coating, which is applied onto the fibrous facing of the gypsum panel after the board has been prepared, acts as both a liquid and vapor barrier (vapor permeance of about 1.2 perms (ASTM E-96)).
The coating is formed from an aqueous coating composition comprising from about 15 to about 35 wt. % of resin solids, about 20 to about 65 wt. % of filler, and about 15 to about 45 wt. % of water, applied to obtain a solids loading of at least about 50 lbs. per 1000 sq. ft, such as about 110 lbs. per 1000 sq. ft. A preferred resin for use according to this patent is a latex polymer that has been sold by Unocal Chemicals Division of Unocal Corporation under the mark 76 RES 1018. The resin is a styrene-acrylic copolymer that has a relatively low film-forming temperature.
In accordance with the teachings of the '631 patent, the aqueous coatings composition formed from the resin is not applied to the fibrous mat-facing of the gypsum board until after the board has been prepared. The post-applied coating is dried effectively at oven temperatures within the range of about 300° to 400° F. If desired, a coalescing agent can be used to lower the film-forming temperature of the resin. Since the bond between the fibrous mat and board core is formed during board preparation, the post-applied coating does not affect the strength of that bond and the board is able to satisfy it supporting function just as do the boards of the '496 patent.
More recently, a coated fibrous mat-faced gypsum board of surprisingly effective moisture resistance, having a predominantly inorganic coating on the mat, was developed, see U.S. Published Application 2002/0155282, which is incorporated herein by reference. The mat used to prepare the gypsum board described in this published application was pre-coated with a predominately inorganic coating containing a minor amount of an organic binder and the pre-coated fibrous mat then was used as one of the facers in the manufacture of a gypsum board. Surprisingly, the coating on the pre-coated mat had sufficient porosity to allow water vapor to permeate through the mat during manufacture of the board, but provided the board with unexpectedly effective moisture resistance.
Using a pre-coated mat to manufacture the board significantly simplified the manufacture of the board. The coating was comprised of a mineral pigment (pigmented filler material), an inorganic binder and a latex polymer binder. In particular, the coating comprised a dried (or cured) aqueous mixture of a mineral pigment; a first binder of a polymer latex (organic) adhesive and, a second binder of an inorganic adhesive. On a dry weight basis, the first polymer latex binder comprised no more than about 5.0% by weight of the coating, and the second inorganic binder comprised at least about 0.5% by weight, of the total weight of the coating.
Because the board described in this published application was designed to be used in situations where the surface of the board is not required to support the attachment of tiles, insulating material or other construction materials, e.g., exterior finishing materials, the strength of the bond between the pre-coated mat and the gypsum core was of limited importance. Indeed, the board of this published application has not been used in such situations and the bond between the pre-coated mat and the gypsum core in the disclosed board would not inherently have the necessary tensile strength for such uses.
A manufacturer of a pre-coated mat of the type used in making the gypsum board of the U.S. Published Application 2002/0155282 recently published its own application, U.S. Published Application 2003/0134079 directed to producing an improved coated glass mat using substantially the same coating formulation. The '079 published application is directed principally to an improved procedure for making a coated mat purportedly having improved tensile (tear) properties. The '079 application suggests using the pre-coated mat for making a gypsum panel product and speculates about mat characteristics desirable for that specific application.
In particular, the '079 published application speculates that there is a fine balance between the portion of coated mat and portion of uncoated, exposed glass fibers in the mat needed to make the pre-coated mat suitable for making a gypsum panel product. As described, if too much glass mat is exposed such that the gypsum cannot cover it entirely, the application speculates that the resultant board is unacceptable. Conversely, if not enough glass fibers are left exposed, the '079 application indicates that the resulting board also is not acceptable. In both cases, the '079 application speculates the board may fail a flexural stress test.
The '079 application goes on to suggest that a proper balance is obtained when the coating penetrates into the glass mat substrate to a depth of from 25 percent to 75 percent of the thickness of the coated glass mat. The '079 application does not address the situation where the gypsum board in intended to be used in circumstances where it is required to support a substrate such as tiles, insulating material or other exterior finishing materials.
The present inventors have determined that the coating parameters postulated by the applicants of the '079 application are not suitable for producing a gypsum panel, using current commercial manufacturing techniques, that can be used in situations where tiles, insulating material or other exterior finishing materials must be supported by the panel. In particular, unless the extent of coating penetration in the pre-coated mat is controlled within a much smaller window and the process conditions for subsequently making the gypsum board are properly observed, the resulting bond between the pre-coated glass mat and the gypsum core does not have the required tensile strength for supporting tiles, insulating material or other construction materials, such as exterior finishing materials.